The
softshell turtles of the family Trionychidae have highly derived morphology
evolved to adapt the turtles to an almost entirely aquatic environment. These
adaptations include a smooth leathery skin covering with a reduced bony shell, a
flattened body shape, and heavily webbed toes. Thirty-one species in 13 genera
are distributed in Africa, Asia (including New Guinea), the Mediterranean, and
North America. The fossil record supports the family’s former presence in
Australia, Europe, and South America. Trionychid phylogenetic relationships are
a fairly well resolved and robust molecular phylogeny of trionychids has been
established. And, species boundaries within a number of widely distributed
species or species complexes have been clarified. However, to date the
taxonomic status of the critically endangered Shanghai softshell turtle, Rafetus swinhoei, is still a matter of
debate. Ranked as one of the 100 most endangered species (only four live
individuals of this species are known, two in Vietnam and two in China). Historically,
this species had a large distribution range, including the Yellow River, Yangtze
River, and their tributaries in China and the Red River system, as well as Ma
River and associated wetlands in Vietnam. Previous molecular and morphological
comparisons suggested that this is a single species but more recent work produced
radically different results, and described populations in Vietnam as a new species,
R. vietnamensis. A subsequent study
shed doubt on this view by highlighting sources of potential errors. Despite
this, it is likely that populations from Vietnam and China constitute independent
evolutionary lineages given the distance and river systems separating them.
To
test this hypothesis, Minh Le and
colleagues analyzed data from two mitochondrial loci (cytochrome b and ND4) and
one nuclear intron (R35) for all trionychid turtle species, except Pelochelys signifera, and for all known
populations of Rafetus swinhoei in
Vietnam and one from China. Phylogenetic analyses using three methods (maximum
parsimony, maximum likelihood, and Bayesian inference) produce a well resolved
and strongly supported phylogeny. The results the time-calibration and
biogeographic optimization analyses show trionychid dispersed out of Asia between
45 and 49 million years ago in the Eocene. Interestingly, the accelerated rates
of diversification and dispersal within the family correspond surprisingly well
to global warming periods between the mid Paleocene and the early Oligocene and
from the end of the Oligocene to the mid Miocene. The study also indicates that
there is no significant genetic divergence among monophyletic populations of Rafetus swinhoei, and that previous
taxonomic revision of this species is unwarranted.
Citation
Le,
M., Duong, H. T., Dinh, L. D., Nguyen, T. Q., Pritchard, P. C., &
McCormack, T. (2014). A phylogeny of softshell turtles (Testudines:
Trionychidae) with reference to the taxonomic status of the critically
endangered, giant softshell turtle, Rafetus
swinhoei. Organisms
Diversity & Evolution, 1-15.